1. Field of the Invention
This disclosure relates to radio frequency (RF), microwave and millimeter systems, and more particularly, to circuit topologies for frequency mixers that can be used in such systems.
2. Description of the Related Art
Frequency mixers—i.e., circuits that convert a signal modulated at one frequency to a signal modulated at another frequency—may be critical circuit components in certain wireless communication systems, such as RF, microwave and millimeter-wave systems. FIG. 1 shows a conventional switching or commutating mixer 100, also known as a double-balanced field-effect transistor (FET) mixer. Switching or commutating mixer 100 is also called a passive mixer, as 0 Hz (“DC”) bias or quiescent current does not flow through the FETs in the mixer. Double-balanced FET mixer 100 of FIG. 1 can provide both frequency up-conversion and frequency down-conversion. Of the two input and output signals at the mixer's input port and output port, across terminals 101a and 101b and across terminals 102a and 102b, respectively, the higher frequency signal is usually referred to as the RF signal, while the lower frequency signal is usually referred to as the intermediate frequency (IF) signal. In FIG. 1, the RF signal and the IF signal may each be a differential signal. Differential local oscillator (LO) provides a differential oscillator signal across terminals 103a and 103b at the LO frequency to control the frequency conversion performed by mixer 100.
For many frequency mixing applications, input and output signals that are single-ended are preferred over differential signals. FIG. 2 shows conventional double-balanced FET mixer 200 with RF and IF signals (provided at input port 201 and output port 202, respectively) that are single-ended. The RF signal and the IF signal can each be converted to a differential signal using a balun (e.g., balun 210 or balun 211, as shown in FIG. 2). The operating frequency range of mixer 200 of FIG. 2 may be limited by the bandwidths of baluns 210 and 211. For a FET mixer with integrated baluns, the size of each integrated balun may be limited by the chip area, which may restrict the IF balun operation to frequencies above a few hundred megahertz. There are many applications, however, that require the mixer to operate at lower IF frequencies, even down to DC. For example, when used as a phase-detector, an IF output signal that is modulated down to DC may be required.
FIG. 3 shows a conventional double-balanced diode mixer 300 that has an IF frequency response that reaches down to DC. While double-balanced diode mixers can achieve IF frequencies down to DC, double-balanced FET mixers are generally preferred. This is because double-balanced FET mixers are easier to integrate onto a monolithic semiconductor die. Furthermore, double-balanced FET mixers may offer better impedance-matching over wide ranges of bandwidths, higher isolation, and lower local oscillator power demands than double-balanced diode mixers. Designing a double-balanced FET mixer with a single-ended IF terminal is, however, challenging.
FIG. 4 shows conventional double-balanced FET mixer 400 with an IF frequency response that reaches down to DC. In FIG. 4, the IF signal path through the IF transformers T2 and T3 may allow for a low IF frequency capability. (See, e.g., U.S. Pat. No. 6,957,055, entitled “Double Balanced FET Mixer with High IP3 and IF response Down to DC levels” to D. Gamliel.) The parasitic reactances of transformers T2 and T3, however, may adversely affect both the impedance-matching and the high frequency performance of IF port 102. Also, the conversion gain of double-balanced FET mixer 400 may be degraded by the RF signal loss through these transformers.